Are you referring to the HT 2-stroke engine?
If so, you may need to adjust the clutch spring inside the cases to a lighter setting.
Not too light mind you, you don't want the clutch to slip.
Here's a link to Norm's tutorial on how to tighten the spring. Obviously you would turn the sprocket the other way to loosen it.http://motorbicycling.com/f39/adjust...sion-8392.html
Also, have you tried to round the edge of the clutch cam that presses onto the bucking bar?
Just remove the sharp edge, not much, and make it very smooth so it will stop digging into the end of the bucking bar.